The typical integrated circuit is designed to connect to a single external supply voltage. Within the integrated circuit several internal voltages are often used for example to bias the substrate, to power subcircuits, and to charge circuitry having distributed and parasitic capacitance. The integrated circuit design, therefore, includes voltage regulators for developing the internal voltages from the single externally supplied voltage. Each voltage regulator operates at a given efficiency, usually in the range of 50 to 80 percent. The losses inherent in less than 100 percent efficiency are expended as heat. In addition, some conventional voltage regulator circuits require continuous current flow for establishing bias voltages. For integrated circuits including dynamic random access memory, for example, the current drawn from the external voltage supply when the circuit is idle is almost entirely the bias current required for voltage regulation.
FIG. 2 is a functional block diagram of a voltage regulator circuit for a conventional integrated circuit. The externally supplied voltage VCCX on line 142 is input to an initial regulator 110. Initial regulator 110 includes a series pass regulator designed to drive high impedance loads. Initial regulator 110 is coupled to preregulator 112 and VCC regulator 120. Preregulator 112 and VCC regulator 120 include series pass regulators that are physically larger than the regulator used in initial regulator 110. These larger regulators more efficiently pass larger currents to lower impedance loads than the smaller regulator of initial regulator 110. However, as a consequence of the ability to regulate larger currents, these larger regulators dissipate more energy as heat and require larger bias currents than a smaller regulator. From 50 to 98 percent of the total standby power dissipation of a conventional integrated circuit dynamic memory device is dissipated by the circuit performing the function of preregulator 112. Preregulator 112 in the conventional circuit must be carefully designed.
The design of preregulator 112 is constrained by several design goals. Preregulator 112 should have low impedance drive capability to support rapid changes in load current supplied to charge pump 118. Second preregulator 112 should be located in an area of the integrated circuit where power dissipation in the form of heat can be tolerated. As an analog circuit, preregulator 112 should be located in an area of the integrated circuit away from digital circuitry so that interference between the analog and digital circuits is minimal. Coupling of substrate currents is one example of unintended coupling that can be reduced by physically separating analog and digital circuits.
In the conventional design of preregulator 112, sophisticated circuitry is employed to avoid underdamped output voltage to charge pump 118 when supply voltage VCCX or load current to charge pump 118 changes suddenly. Such circuitry conventionally takes the form of an operational amplifier requiring large bias currents (for example 5 milliamps) to support a large series pass transistor (for example 10,000 microns by 1.5 microns). The complexity of the operational amplifier adds to space requirements for preregulator 112 and adds to the bias current requirement, the power dissipation, and the heat generated by preregulator 112.
To assure accurate performance of the operational amplifier, conventional integrated circuit design techniques require additional mask steps and tighter tolerance controls for the fabrication processes involved as compared to the mask steps and process controls required for the remainder of the integrated circuit. Additional mask steps and process controls add to the expense of integrated circuit manufacture, increase fabrication time, and decrease yield.
Some applications require integrated circuits having low operating and low standby power consumption. In applications, including for example, portable computers, low power consumption is desirable while operating and highly desirable while portions of the computer are idle. The summation of the bias current drawn by each integrated circuit in the computer is a significant continuous current drain on the battery. Bias currents, therefore, contribute to limited battery operation, contribute to increased cost for sufficient battery supplies for particular applications, and make some applications infeasible.
Thus, there remains a need for an integrated circuit design having improved voltage regulation circuitry characterized by lower bias current consumption. A design is needed for providing regulated voltage to the subcircuits of an integrated circuit while requiring less bias current, while occupying less area of the integrated circuit, while dissipating lower power and generating less heat, and while responding to abrupt changes in supply and load conditions without generating underdamped output voltage.